Mercury-arc tube with grid control



Oct. 15, 1929. J. SLEPIAN 1,731,537

MERCURY ARC TUBE WITH GRID CONTROL Filed Oct. 15, 1925 ZZZ/l,

I WITNESSES: v u a INVENTOR 1,7 JoJep/z S/ep/an. 3, My \LXSZQU BY 27m TORNEY Patented Get. l5, 1929 UNITED STATES PATENT OFFICE JOSEPH SLEPIAN, OF WILKINSIBURG, PENNSYLVANIA, ASSIGNOR TO WESTINGHOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIA MERCURY-ARC TUBE WITH GRID CONTROL Application filed October 15, 1923. Serial No. 668,555.

My invention relates to mercury arc devices and it has special reference to mercury are devices utilizing a grid for controlling the current flowing therethrough.

In the practical application of grids for the control of currents flowing in mercury are devices between a cathode, which is excited to emit a supply of electrons, and an anode, it has been found that an excessively large voltage was necessary to influence materially the flow of current. Very often, the power required for controlling purposes is of the same order as the power of the main working circuit. Accordingly, only slight practical use was made of grid control for mercury are devices, in spite of the otherwise highly desirable operative characteristics of such devices, such as their capacity to handle large power.

The principal object of my invention is to provide an improved grid-controlled mercury arc tube wherein the action of the grid is positive and dependable, and the power required for the control of the current flow is a minimum.

l have found that the apparent inoperativencss of the grid heretofore employed in such devices and the disroportionately small effect of the grid on tiie current flow may be radically improved by a small but, in itself, fundamental change in the construction of the tube. To this end, I provide, in a mercury arc device of the above character, and along the path of the current between the mercury cathode and the anode, a passageway having, in effect, walls of conducting material. A grid, which may have any of the familiar forms known in the construction of electron tubes, spans the conducting walls of the passageway. The grid and the conducting passageway are conductively connected and are preferably maintained at the same potential in accordance with the desired control of the current through the tube.

Essentially, my invention aims to eliminate a zone which is usually found between the grid and the walls of thevessel which is not subjected to the controlling influence of the grid and which thus permits conducting particles such as electrons to slip past the grid without coming into the region of the positive controlling action of the same.

\Vith the foregoing and other objects in View, my invention consists in the constructions and details of arrangement described and claimed hereinafter and illustrated in the accompanying drawing, wherein Figure 1 is a diagrammatic view of a mercury arc tube employed for the rectification of an alternating current and for the supply of the same to a direct-current load;

Fig. 2 is an enlarged sectional view of the rectifier shown in Fig. 1; v

Fig. 3 is a sectional View of a portion of the rectifier shown in Figs. 1 and 2, illustrating the construction of the portions of the tube adjacent the grid;

Fig. 4 is an enlarged sectional view of a portion of the tube adjacent to the grid, explaining the operation of the improved grid construction;

Fig. 5 is'an enlarged sectional view of a portion of a tube with the grid arranged in accordance with the prior-art practice without the employment of my improved construction and illustrating the harmful action which I have succeeded in overcoming;

Fig. 6 is a diagrammatic view of a single anode rectifier equipped with agrid according to my invention;

Fig. 7 is a diagrammatic sectional view of a modified tube which embodies the broad principles of my invention; and

Fig. 8 is a sectional detail view of a portion of the tube shown in Fig 7.

In the system of Fig. 1, a transformer 1 supplies current through a double-wave rectifier 2 to a direct-current load 3. The rectifier 2, which may be also used as a switch for cutting off the power of the supply from the load, is shown in an enlarged view in Fig.

2 and comprises a substantially tubular glass envelope or vessel 4 having a lower portion 5 arranged as a container for mercury 6, serving as a cathode, and two anodes7 which are --80 constitutes a conducting the surface of the mercury 6 and serves to maintain an are upon the surface of the mercury electrode -6 for the emission of electrons during the o eration of the rectifier.

The supp y transformer 1 has a primary.

I provide, furthermore, an arrangementfor controlling the flow of current through the mercury are by means ofv which I may stop the supply of the direct current to the load 3.

To this end, as shown in detail in Fig. 3, a grid 18 is spanned between the walls of the tubular portions 8 of the vessel leadin to the 5 anode 7, the ortions of the walls a jacent the grid 18 being covered with a conduct ng layer or deposit 19, of iron for instance, which is in conductive connection with the grid 18.

The conductive deposit or coating 19, which cylinder, and the grid 18 mounted therewithin are so arranged that all the current-conducting particles moving between the cathode 6 and the anodes 7 pass either through the. openings of the grids or between the metallic walls 19 of the cylinder and the border portion of the grid.- The advantages and the efiect of the above-described grid construction will be explained hereinafter. y

A control source of electromotive force 20 may be connected, by means of a switch 21, between the cathode 6 and the individual grids 18 respectively for applying a negative charge to the grids andstopping the flow of current between the cathode 6 and the anodes 7.

I will'now explain the deficiencies of the grid construction utilized heretofore and the advantages secured by my construction. The high conductivity of mercury are devices employing an arc playing on the surface of the mercury electrode is based on the fact that the negative electrons which are emitted from the are upon the mercury surface move, under the influence of the electrostatic field induced by the potential ap lied to the main electrodes in the direction rom the cathode to the anode, and in their travel they collide with the molecules of mercury vapor which and the electrons thus obtained start, in turn, to travel along the path between the'cathode and the anode, new collisions take place, more 1 carriers of electricity are obtained and the fill the vessel, splitting the same into positive 7 ions and negative electrons. The positive lOIlS abundance of carriers of electricity so obtained produces thehigh conductivity of such the negative charge of the grid is automaticah ly neutralized and taken care of, by the abundant supply of positive ions in the tube.

If the current is, however, made to go out by some means for a short time interval, a negative charge on the grid may prevent the current from again starting to flow. For instance, a negative charge applied to a rid will prevent the current from starting to ow after the interval of no current flow during the reversed half cycle of alternating current.

This action of the grid may 'beexplained as follows. i

A negative charge existing upon a grid prior to the initiation of a current flow repels the negative electrons emitted from the cathode, preventing their passage past the grid; furthermore, if the potential upon the grid be sufliciently high, the velocity of the electrons will not reach a value suflicient to produce large numbers of positive ions by collision with the molecules of vapor in the space between the cathode and the grid. Such positive ions as are formed will be attracted, by the negative charge upon the grid and neutralized therein. It is thus possible to prevent the initiation of current flow in a mercury rectifier tube by the application of a sufficiently high negatlve potential to a grid, although the cathode is maintained in'an electron-emitting state and a proper potential, which would ordinarily cause a'flow of current, is applied between the cathode and the anode. v v In spite of the above, theoretically feasible and apparently true explanation of the operation of a grid, small success has been obtained in the use of grids for the control of mercury are devices. I explain these difliculties in the practical operation of prior grid controlled mercury are devices as follows: During the period of current-conduction,the heavy positive ions filling the entire space of the vessel difi'use partially towards the walls thereof and accumulate also on the insulating walls in the neighborhood of the grid. With an ordi narygrid extending across the pathof the electrons there will exist a. space, be it even very small, between the grid and the adjacent insulating wall of the vessel in which the efi entirel neutralized b the positive charge on the a jacent walls, t us a rding to the electrons a passageway throu h whlch they may slip past the grid into t e part of the vessel between the grid and the anode.

The phenomenon of current conduction past grids of ordinary construction 1s 1llustrated in Fig. 5 showing an enlarged view of the portion of the tublular vessel arm 8 and the grid 18a. With the ordinary construction, there will be one or more spaces 22 be tween the boundary 23 of the grid and the ad- 'acent wall portion 24, said spaces being possily small but nevertheless sufficiently large for an electron to slip throu h in the above described manner, the crosses and dashes indicating the positive charge accumulated upon the walls of the vessel and the negatlve charge applied to the grid, respect1vely.

A negative electron which SllPS thus into the space between the grid 18 and the anode 7 moves thereupon under the full influence of the applied potential, collides with the molecules of the vapor filling the vessel, generates new electrons and ions and in a small fractlon of time the tube becomes fully conductive in zones in the neighborhood of the grid and to I this end the grid is mounted within a portion of the tube. having the inner walls thereof covered with a layer 19 of conducting material such'as iron or nickel, as shown in Figs. land 2. The grid 18 is pressed into the tubular conducting layer 19 of the vessel and makes contact with the conducting walls thereof at one or more points.

The conditions prevailing upon the application of a negative charge to the grid, arranged according to the improved construction, are shown in Fig. 4. With the improved arrangement, the walls surrounding the grid are charged to the same potential as the grid, and exercise, upon the electronstending to move occasionally towards the anode, a repelling action in the same mannervas the grid. By my improved arrangement of the grid, I positively prevent the neutralization of the negative charge which is applied to the grid and I also increase the efiectiveness of the grid action as such.

To illustrate the effectiveness of a grid constructed according to my invention, the following results of tests, made with a tube having first an ordinary grid arrangement and then an improved grid arrangement, may be compared. The cathode of a mercury arc tube was excited by means of an auxiliary arc and a negative potential of 150 volts was applied to the grid with respect to the cathode. Under those conditions a potential of 42 volts applied to the anode was suflicient to start a current through the tube, whereas with zero voltage applied to the grid it required 17 volts on the anode to start a current.

With the same kind of tube, but provided with an improved grid arrangement, it was sufiicient to apply to the grid :1 negative potential of 6 volts only to prevent a current through the tube with the anode maintained at a positive potential of 220 volts. The latter figures do not, in any way, indicate the limit ofthe possible performance of such tubes and are given only in order to illustrate the effectiveness of the new grid arrangement.

It was the sinall change in the grid construction which turned a mercury arc tube, that was heretofore incapable of economical and rational control by a grid, into an instrument whose operation may be controlled by a small energy applied to the grid circuit.

In Fig. 6 is shown a mercury arc tube, having a mercury cathode 6 and a single anode 7 mounted in a tubular glass vessel 25. A grid 18' is provided in the tube and is constructed in accordance with theprinciples set forth hereinabove and comprising a tubular metallic passageway 19 formed by any convenient method, by spraying of a layer of iron for instance,or inserting an iron cylinder fitting into the tube, and a grid 18 supported by the metallic walls of the passageway and in conducting connection therewith. I have foundthat the operation of the tube may be very effectively controlledby a small voltage which is applied to the grid in the same manner as the double anodevtube described in Figs. 1 and 2.

In Fig. 7 is shown a modification of my invention wherein the metallic cylinder 19 surrounding the grid 18" forms a part of the walls of the glass vessel 25' and is joined thereto by means of gas-tight welds. The particular construction of the metallic cylinder and of the metal-to-glass welds are not a part of my present invention and are described and claimed in the copending applications of C. Kirwer Serial No. 665,487 and D. Ulrey Serial No. 665,407, and D. Ulrey and C. Kirwer Serial No. 665,414, all filed on September 28, 1923 and assigned to the vestinghouse Electric & Manufacturing Company.

The construction shown in Fig. 7flisillustrated in detail in Fig.8. The metal cylinder 19" is made of copper and has two end portions provided with knife-like edges 26 and 27 to which the adjoining glass portions are welded. A grid 13 is mounted between the walls of the cylinder, the latter being coated with a mercury-resistant material 27 such as nickel, in order to protect the copper from the detrimental action of the mercury.

The grid constructions shown and described hereinabove may be made in any of the familiar forms known in the construction of grid-controlled vacuum devices and any of these forms may be used in connection with my invention, which aims to prevent a neutralization of the negative or controlling charge upon the grid'by an accumulated, un

controlled charge on portions of the vessel in the neighborhood of the grid, and it is my desire that any means which intend to produce the same etfectshallibe regarded as covered by the appended claims. Likewise, I wish that the forms of my invention illustrated and described hereinabove shall be considered as lustrative of the mode in which the same may be practically applied and I desire, therefore, that the appended claims shall cover all such modifications as come within the true scope and spirit of my invention.

I claim as my invention:

1. A gaseous electric device comprising a vessel, two main electrodes in said vessel, a gaseous medium for conducting current between said electrodes, the conducting path between said electrodes including a passageway having conducting walls, a grid disposed in said passageway, means sealed through the tube wall for controlling the electric potential of said grid and the electric potential of said conducting walls.

2. A mercury arc device comprising an evacuated glass vessel, a mercury cathode in said vessel, an anode in spaced relation to said mercury cathode, a portion of said glass vessel constituting a tubular passageway for a flow of current, a coat'of metal on the walls constituting said passageway, and a grid spanning said passageway between the metallically coatedwalls thereof and means sealed through the tube walls for providing a conductive connection to said coat of metal,

3. A 's'pace current device comprising a'ves-v sel of insulating material, a cathode and an anode mounted 1n said vessel, a gaseous medium for conducting current between sa1d cathode and said anode, and a grid spanning the entire cross-sectional area of sald vessel .over substantially the entire cross-scction of said tubular passageway, a tubular layer of conducting material substantially in contact with the inner surface of-said tubular passageway and extending longitudinally along said passageway in both directions from said grid, and adapted to'beconductively connected to' the negative terminal of a source of JOSEPH SIFJEPIAN,

device comprising a 

